Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States.
J Org Chem. 2011 Oct 21;76(20):8406-12. doi: 10.1021/jo201624p. Epub 2011 Sep 26.
The method of continuous variation (often referred to as Job's method) is an easy and common method for the determination of the reactant stoichiometry of chemical equilibria. The traditional interpretation of Job plots has been limited to complex association equilibria of the type nA + mB ⇌ A(n)B(m), while little focus has been placed upon displacement type reactions (e.g., A + B ⇌ C + D), which can give Job plots that look quite similar. We developed a novel method that allows the user to accurately distinguish between 1:1 complex association, 2:2 complex association, and displacement reactions using nothing more than a pocket calculator. This method involves preparing a Job plot of the system under investigation (using regularly spaced mole fractions), normalizing the measured quantities (such as the concentration of A(n)B(m) or C for the above reactions) to their maximum value (i.e., at mole fraction 0.5), and determining the sum of the normalized values. This sum is then compared with theoretically predicted normalized sum values that depend on the nature of the equilibrium. The relationship between, on the one hand, the sum of the normalized values and, on the other hand, the reaction equilibrium constant and the concentration of the stock solutions used for the preparation of the Job plot is also explored. The use of this new technique for the interpretation of Job plots permits users to readily determine information that can be obtained otherwise only with laborious additional experiments, as illustrated by the analysis of four Job plots taken from the literature.
连续变化法（常称为乔布法）是一种测定化学平衡反应物化学计量比的简单而常见的方法。传统的乔布图解释仅限于复杂的缔合平衡，如 nA + mB ⇌ A(n)B(m)，而很少关注置换类型的反应（例如，A + B ⇌ C + D），这些反应也可以给出看起来非常相似的乔布图。我们开发了一种新方法，用户只需使用袖珍计算器即可准确区分 1:1 复杂缔合、2:2 复杂缔合和置换反应。该方法涉及制备所研究体系的乔布图（使用规则间隔的摩尔分数），将测量的量（例如上述反应中 A(n)B(m)或 C 的浓度）归一化为最大值（即摩尔分数为 0.5），并确定归一化值的总和。然后将该总和与取决于平衡性质的理论预测的归一化总和值进行比较。另一方面，归一化值总和与反应平衡常数和用于制备乔布图的储备溶液浓度之间的关系也进行了探讨。该新技术用于乔布图的解释，使用户能够轻松确定其他情况下只能通过繁琐的额外实验才能获得的信息，如从文献中提取的四个乔布图的分析所示。
